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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

.

441 USER COMMENTS

Greg, you will have to wait until the curb is completely cured before adding another material. Sakrete Top ‘N Bond can be used to finish the curb if the repair requires up to a ½”. Do NOT use a bonding agent with Top ‘N Bond as it will disturb the integrity of the product.
- Chris Technical Services
Tuesday, March 31, 2015 at 2:31 PM

I built a shower curb made with a bag of sand toping mix. The curb is out of square and not level. I need to chip away or add about a half inch to the side of the curb to level it out. What product do I use and what is the process? Thanks in advance for your help.
- Greg
Sunday, March 29, 2015 at 5:08 PM

Chris, yes this can be done with Sakrete Sand Mix and Sakrete Bonder and Fortifier. Sand Mix can be used from 2” down to ½” and the Bonder and Fortifier will help bond the new material to the existing concrete. You can use the Bonder and Fortifier as a primer by simply using a paint brush to lightly coat the area first. This can also be used as a 50/50 admixture with water to the Sand Mix for a stickier mix. If you need less than a half inch use Sakrete Top ‘n Bond from ½” down to a feather edge. DO NOT use Bonder and Fortifier with Top ‘n Bond.
- Chris Technical Services
Wednesday, March 25, 2015 at 8:33 AM

Jim, make sure to first properly prepare the surface by removing any bond breakers such as paint, stain, loose material, dirt, dust, etc. Sakrete Sand Mix is used for repairs from 2” down to a ½”. If you are pouring more than 2” we suggest using Sakrete High Strength Concrete Mix, Sakrete Maximizer, or Sakrete 5000 Plus. Adding 4” to a slab will require a lot of material and depending on how large the area is may require a pre-mixed truck. Keep in mind the cracks may be due to movement and may reappear. I suggest resurfacing the slab with Sakrete Flo-Coat. Flo-Coat can be used in a flowable consistency and is perfect for larger areas. Flo-Coat can be applied from ½” down to a feather edge. Mix to a stiffer consistency to fill in the low spots prior to resurfacing up to ½”.
- Chris Technical Services
Tuesday, March 24, 2015 at 12:32 PM

I have a four by eight covered and walled area with a perfectly sound concrete base. It's a trash bin storage area. Problem is it extends to a patio which pitches toward the area and water pools there. I was thinking of building a frame that pitches back towards the patio so the area will stay dry. This would effectively create a small curb and patio water would simply drain to the grass. Obviously the area gets little foot traffic and it's in south so no chance of hard freezes. Can I do it? How thick and which product?
- Chris
Monday, March 23, 2015 at 10:25 PM

I have a 40 year old garage slab that has broken, cracked, and dropped in places. Scaling is a problem as well. I used patching material on the cracks several years ago and it worked fairly well. This brutally cold winter, however took it's toll and all the cracks, plus more have returned. I would like to pour a new 2 to 4 inch floor over the old one. I know that tearing the old one up is the best option but it is not a good financial option at this time. What would my best bet be, as a second option, to help the new concrete adhere to the old?
- Jim
Thursday, March 19, 2015 at 1:54 PM

Leon, when applying tile you want a roughened surface to start with so that your thin set has something to grab onto. Be sure before installing your tile the surface is sound and not brittle.
- Chris Technical Services
Friday, March 6, 2015 at 4:06 PM

Bob, Sakrete Bonder and Fortifier is appropriate to use for this application and will add bond strength between the footing and mortar.
- Chris Technical Services
Friday, March 6, 2015 at 3:59 PM

Had a concrete company put a 1 1/2 cap in living room notice large hollow spots. They told me it's normal and won't affect tile floor I'm getting ready to lay. Seems like I have a problem what should I do
- Leon
Friday, March 6, 2015 at 7:28 AM

I have recently placed a reinforced concrete footer in a patio for placement of brick pavers on top of the footer. Do I need to use some type of bonding agent on top of the footer before placing down a 1/2" mortar bed for my brick? Any help would be appreciated. Thanks.
- Bob
Friday, March 6, 2015 at 3:01 AM

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